The present invention relates to a ceramic heater and a glow plug equipped with the ceramic heater.
There are conventional ceramic heaters used for glow plugs and the like. Such ceramic heater contains a basal body made of an insulating ceramic (e.g., silicon-nitride-based sintered body) and a heating element (containing a conductive component (e.g., WC)) embedded in the basal body. Such ceramic heater further contains a lead wire (made of W or the like) of which end portion is embedded in the heating element. With this, the heating element is electrically connected with electrodes for energizing the heating element. The heating element is prepared by adding a conductive material (e.g., WC) to a silicon-nitride-based material and a sintering aid, then forming the resulting mixture into a compact, and then sintering the compact into the heating element. It is, however, difficult to sinter the silicon-nitride-based material and the conductive material. Thus, it may be difficult to obtain a heating element that is sufficiently compact, if the sintering aid is in a normal amount. Japanese Patent Laid-open Publication JP-A-8-64346 teaches a ceramic heater prepared by adding a relatively large amount of a sintering aid in order to sufficiently produce a liquid phase during sintering.
If a relatively large amount of a sintering aid is used in the production of a ceramic heater, an excess of the sintering aid may not be uniformly dispersed, but may form a condensed zone partly dispersed. This condensed zone has a higher concentration of the sintering aid, as compared with that of the remainder, and tends to occur at a periphery of a lead wire""s end portion embedded in the heating element. Therefore, it may be necessary to improve strength of the vicinity of this end portion.
It is therefore an object of the present invention to provide a ceramic heater that has a sufficient bending strength even at its connected portion at which its lead wire""s end portion is embedded in its heating element.
It is another object of the present invention to provide a glow plug equipped with such ceramic heater.
It is still another object of the present invention to provide a process for producing such ceramic heater.
According to the present invention, there is provided a ceramic heater extending in an axial direction to have an elongate shape. This ceramic heater comprises (a) a basal body; (b) a lead wire embedded in said basal body; and (c) a heating element embedded in said basal body. This heating element comprises (1) a matrix ceramic phase; (2) conductive ceramic particles dispersed in said matrix ceramic phase; (3) a portion in which an end portion of said lead wire is embedded; (4) a reference zone defined on a cross-section of said ceramic heater, said cross-section being defined as being perpendicular to said axial direction of said ceramic heater and as being disposed at a center of said end portion of said lead wire, said reference zone being away from said end portion of said lead wire by a distance of 40 xcexcm, or greater; and (5) a condensed zone optionally contained in said heating element and having a thickness of 0-5 xcexcm, said condensed zone being defined on said cross-section of said ceramic heater and defined as being a zone such that a concentration of an element contained in said matrix ceramic phase of said condensed zone is two times or greater an average concentration of said element contained in said matrix ceramic phase of said reference zone, said average concentration being defined in a direction along a thickness of said reference zone.
According to the present invention, there is provided a glow plug equipped with the ceramic heater.
According to the present invention, there is provided a process for producing the ceramic heater. This process comprises (a) providing a first precursor of said heating element, said first precursor comprising a first weight percent of a rare-earth element; (b) embedding said end portion of said lead wire in said first precursor to form a first precursory body; (c) embedding said first precursory body in a second precursor of said basal body to form a second precursory body, said second precursor comprising a second weight percent of a rare-earth element, a ratio of said first weight percent to said second weight percent being 0.5 or less; and (d) sintering said second precursory body into said ceramic heater.